High frequency wattmeter



May 6, 1941.

E. MITTELna/Julu4 HIGH FREQUENCY WATTMETER Filed April 22, 1938 fag! URIIl U 25 Patented May 6, 1941 HIGH FREQUENCY WATTMETER Eugen Mittelmann,Vienna, Germany Application April 22, 1938, Serial No. 203,661 InAustria April 26, 1937 4 Claims.

This invention relates to a method of and apparatus for the measurementof power which is absorbedin the output circuit of a high frequencygenerator, and while my invention is applicable to the measurement ordetermination of the power which is absorbed or utilized in the outputcircuit of whatever purpose for which said output is utilized, theinvention has special application to .the determination of the powerabsorbed by thel patient in connection with the treatment thereof byshort wave or high frequency therapy.

The object of my invention is to provide a method of and apparatus forthe determination of a component of the total power absorbed in anoscillatory circuit, which component can be referred to as that portionof the total power which is useful for a specic purpose as distinguishedfrom that which constitutes losses in the apparatus itself or due toother factors.

In the application of my invention to short wave therapy where only afraction of the total high frequency power is absorbed by the patientunder treatment, the remamder constituting radiation losses, leakagelosses, etc., my object is to measure the power absorbed by the patientindependently of total power in the patient's circuit, whereby it can beaccurately known what component or factor of the total power is utilizedin the treatment of the patient.

My invention will be readily understood by reference to the accompanyingdrawing wherein is illustrated diagrammaticaliy the output circuit of ashort wave or high frequency transmitter or generator embodying myinvention.

Referring to the accompanying drawing,

Fig. 1 is a diagrammatic view of the secondary or output circuit of ahigh frequency generator, showing the instrumentalities which I utilizein carrying out my invention; and

Fig. 2 is a similar view to Fig. 1 in which a modified form ofinstrumentality is utilized.

In both of the figures of Vthe drawing, A represents a source of highfrequency oscillations coupled to a secondary or output circuit B, bymeans of a transformer C, all in a manner well known in the art. Thesource of high frequency A may be of any type well known and sui-tablefor the purpose since my invention is not concerned with said apparatusas such.

The oscillatory output circuit B is shown as including the twoconductors I and 2 extending from the secondary of the transformer C andterminating in sui-table electrodes 5 which are spaced apart, andbetween which the object 6 is positioned, which object 6 represents theload which is to be measured as to its power absorption.

If the high frequency is to be applied as treatment to a patient, thenit is to be understood that the object 6 is intended to represent thepatients body or some portion thereof positioned between the electrodes,as is well known and generally understood in this art. The outputcircuit in apparatus of this character generally `has a variablecondenser 4 bridged across the deflection of the pointer is proportionalto the square of the voltage or current. Any standard or well knowndevice of this type may be utilized. That which is shown in Fig. l is ofthe hot wire thermocoupie and galvanometer type, while that shown inFig. 2 is of the dynamometer type having a moving coil and an excitingcoil and utilized in connection with an electronic tube operating in thesquare portion of its characteristic.

In Fig. 1 the thermocoupie 9 is bridged across the output circuitconductors I and 2 by means of conductors 1-1, which include thecondensers 8 8.

The thermocoupie 9 is connected to the terminals of the galvanometer I2by means of conductors I0-III in the usual manner in instruments of thischaracter. The galvanometer I2 has a pointer I3 which moves over acalibrated dial I2a, the deflection of the pointer being proportional tothe square of the voltage in the circuit including the thermocoupie. Thedial is calibrated in equal divisions and in the present instance I haveshown the dial as calibrated in divisions from 0 to 100. v

In accordance with my invention, I provide a shunt circuit around thegalvanometer, which shunt includes a variable resistance I 4 for thepurpose of varying the sensitivity of the galvanometer. This shuntincludes the conductors I5 which are connected to lthe terminals of thegalvanometer and one of which is connected to a terminal of theresistance I4. The other conductor is connected to a contact member I6movable over the resistance I4. Alongside the path of the contact memberI6 is a scale or dial I1 which is calibrated preferably in divisionsfrom 0-100, corresponding to the dial of the galvanometer, and thecontact member carries a pointer I8 which cooperates with the dial toindicate the resistance value by its position relative to the dial.

Since it is desired that the shunt resistance I4 affect the sensitivityof the galvanometer in certain mathematical ratios, as will hereinafterbe explained, it is necessary in the first instance to calibrate thedial I'I of `the resistance in accordance with that ratio rather thanhave the dial II read true values of the resistance Il. It is adjustedso that regardless of what the resistance may happen to be, thatresistance is correct for adjusting the galvanometer to read a value forpower absorbed by the body in the cirsuit when the reading of thegalvanometer pointer I3 on its dial I2a and the reading of the dial I1are the same under certain preliminary conditions defining a typicalratio for a given electrode condition. It will be seen, from laterdiscusson, that the ohmage gradient of the resistance is convenientlyconstructed so as to make the division of the dial I1 linear.

The principle upon which this measurement of the power absorption of theobject is accomplished by the above described apparatus is explained bythe following equations.

The secondary circuit B of Fig. 1 may be denoted byl two equivalentcircuits: (1) a simple coil in parallel with a variable condenser and asingle resistance R0 and (2) the same circuit with a second resistanceR1 in parallel with Ro.

The iirst equivalent circuit represents the secondary circuit B with noload,or the object E not in position between the electrodes .fi- 5.Under these conditions, certain reactive and resistive elements areintroduced into the circuit B and they make up circuit losses, i. e.,leakage losses, radiation losses, all absorptive of power from thecircuit at no load for any given electrodes. Since the circuit is atresonance and all reactive factors are balanced, lthe total powerabsorbed at no load is a function of the constant Ro (for any givenelectrodes), Ro representing the equivalent parallel circuit lossresistance. The resonant voltage developed in the secondary circuit iseo (no load), and, of course, the relation between eo and Ro is:

eo=k Rn (1) where 7c is a comparative factor. In other words, atresonance, the voltage is proportional tothe parallel circuit resistanceor under no loa-d conditions to the parallel equivalent circuit lossresistance.

With the introduction of a body into the circuit and between theelectrodes, more reactive and resistive elements are coupled cinto thecircuit. The circuit is once more tuned to resonance, resulting in a newvoltage e1 and compensating for reactive effect of the newly addedelements. Hence in circuit 2 no new reactive elements are present. Thisleaves the'resistive element introduced by Vthe body 6. In effect, wehave added the so-called equivalent parallel patient resistance to thecircuit denoted by R1.

Again, we see that the voltage e1 is proportional to the combinedparallel resistance, or

RRi Ro +R1 where, of course, K is a comparative factor.

From the above two equations, it will be seen that R1=f(e1) or R1 ispurely a function of e1 (4) Keeping .this in mind, and recalling thatthe power absorbed by R1 is that which We desire, and that it dependsupon the voltage of the circuit, we see that if we can measure thevoltage at resonance with a load in position, and if we know theresistance of the load (that is, the equivalent parallel Vresistanceeffect which the load causes to be added to the circuit), we candetermine the power absorbed by the load by the well known powerequation:

where, if E is in volts, R in ohms, P is given in watts.

Since R1 is a pure function of e1 for any condition of no load, i. e.,same electrode leads, etc., we can determine a proportion correspondingto a solution of Equation 3 above involving initial no load voltage eoand any load voltage e1 (for the same power input, of course). Accordingto Equation 3, R1 is going to be proportional to the damping of voltageof said circuit caused by the insertion of a load, i. e., the ratio ofload voltage e1 to the difference between no load voltage eo and loadvoltage e1. Clearly, since Ithe equation holds true for any ratio, R1will remain a pure function of e1 (any voltage) as explained above, andhence, constant for any given load, the no load conditions being set.

Every change of power will vcau-se a change of both no load and loadvoltage, so that conditions of vEquation 3 are always met. Hence, if weset our value corresponding to factor introduced in the power Equation 5by R1 to satisfy the law of Equation 3 upon the resistance I4 (for thepurpose of operating upon the galvanometer reading in accordance withEquation 5 above, as will be hereinafter explained) we can forget aboutit for any power delivered to the output circuit. The meter, ifcalibrated in power units, will read any power absorbed by the same loadfor the entire range of power which the transmitter is capable ofproducing or within the range which the meter can read.

It is understood that the value R1 can never be actually measured orknown, and that the values e1 or eo need not be metered. All that isnecessary is to find the ratios in a manner to be described hereinafterand to set this up as a factor inuencing the value of resistance I4. Theimport of this is that the measurement of power absorbed by the body 8becomes a simple expedient. By merely determining the ratio representedby the constant value Ri from any damping caused in the voltage by theaddition of/ where P1 is the power labsorbed by the body 6/ at a powerinput such as to give any circuit voltage E, R1 representing theequivalent loss resistance which will be constant for that particularload regardless of power absorbed.

Practically, in my invention all measurements are made by means of a hotwire thermocouple circuit or any similar square law measuring device.

In the galvanometer l2, thedeiiection represented by the pointer I3thereof is proportional to the voltage measured and the sensitivity ofthe galvanometer or --KgEgo' (7) where equals deflection and Kg equals aconstant factor depending upon physical constance of the galvanometer;Eg equals/voltage of the galvanometer circuit; r equals sensitivity ofthe galvanometer. Since we know that R1 is a pure function of e1, and isa constant quantity representing a percentage voltage damping of circuitcaused by the insertion of a known load, if we adjust the sensitivity afor that given body, or load, to be proportional to the reciprocal ofthe equivalent parallel resistance of that body, or

where K' is of course a comparative factor. Then, substituting our R1values back into Equation 7, we get The galvanometer, we know, reads adeflection proportional to the circuit voltage squared; hence, Eg isproportional to E2 or E2 K Rl Comparing this with Equation 6 above, weimmediately see that the galvanometer now reads a quantity proportionalto P1, the power absorbed by the load. l

All that remains, therefore, is the calibration of the galvanomefter toread exactly in watts, and the adjustment of the scale I1 of theresistance I4 to cause the sensitivity of the galvanometer to be v forany given E.

The mechanical procedure of determining the range of ohmage for theresistance I4, is no part of this inventionjgffbut suffice it to saythat the equation for shunting galvanometers to alter the sensitivitythereof may be used for calculation; that is, for any desired value ofa, or

theresistance of the shunt could be determined from the followingequation:

R +R. r--RI (ll) where Rg is the resistance of the galvanometer, Re isthe resistance of the shunt, a is the sensitivity factor. Y

Since R1 is purely dependent on e1, therefore, the factor introduced bythe sensivity adjustment of the galvanometer can be pure operation uponthe voltage read. The setting of the resistance will be controlled bythe voltage damping in the circuit for any ratio fio-81 (see Equation3). This becomes understandableK when comparison is made with Equation1|1 in which, since the shunt resistance Rs is being decreased (see Fig.1 of drawing), the value R1 is exactly proportional to a quantity RR,Hence the required sensitivity may be controlled by variation of theshunt resistance I4 in 'accordance with the damping of voltage for anytwo values eo, e1.

Advantage is taken of this fact in calibration of the scale I1 as willbe described.

The procedure in Calibrating the resistance I4 is as follows:

The output circuit loss without a load or object between the electrodesis first determined in terms of resonance voltage eo, corresponding tothe no-load loss resistance Ru. This is done by tuning the outputcircuit to resonance by means of the variable condenser 4. Thegalvanometer will indicate a maximum Voltage, which by suitable means ofoutput control, for instance by varying the anode voltage of theoscillating tubes of the generator, is adjusted so that the pointer I3indicates the full scale reading, which for illustration We will assumeto be on the scale I2a. This gives a reading on the meter correspondingto en. The value of Rs is now maximum and the scale I1 is caused to readzero. Next a load is placed between the electrodes, which forillustration we assume to be one which will absorb say 60 watts ofpower, and the circuit is again tuned to resonance by the condenser 4.The galvanome'ter pointer will then indicate a lower reading on thedial, say 25. The change in reading now corresponds to the damping ofthe circuit voltage signifying eo-ei and gives the crteron upon which acalibration of the resistance I4 may be based. We now proceed to adjustresistance I4 until the pointer I3 of the galvanometer indicates 60 onthe scale I2a, thus making the galvanometer reading correspond to thatwhich we know is to be the power absorption constant. Hence, any powercan now be supplied to the load by varyingthe voltage E and the cor'rect value thereof will be'read on the dial I2a..

Hav-ing now made this adjustment, the scale I'I is marked at theposition at which the pointer I8 stands. However, the numeral marked onthe scale l1 rls 25 so that it will correspond to the galvanometerreading of 25 which was indicated as corresponding to the value of e1before the resistance was adjusted. This setting corresponds to thedamping of the circuit B caused by the additional introduction of theequivalent loss resistance R1. The instrument will read the correctvalue of power absorption for any arbitrary value of the high frequencyvoltage across the circuit B which mightrbe possibly adjusted by anyknown means of output control, for instance by varying the anode voltageof the oscillating tubes oi the generator A. For any load causing thesame percentage reduction of the resonance voltage eo of the unloadedcircuitl B, there is an infinite number of possible values of powerabsorption correctly indicated by the instrument, corresponding to theamount of high frequency energy whichmight be administered by means ofan arbitrary output control. Having now determined one value of theparallel resistance I4 which changes the galvanometer in a correctreading wattmeter and located it on the dial II of resistance I4, thedial can now be calibrated into proper divisions throughout its range bycomputation and without the necessity of utilizing other known loads.

It is to be understood that the procedure just referred to r4is thatwhich is used in the factory to calibrate the apparatus, and that havingonce calibrated it will read the correct power absorption by the load 6for any arbitrary values of the voltage across the circuit B and theequivalent loss resistance R, respectively.

Having now calibrated the instrument, the

procedure for the measurement of the absorbed power by an object placedin the circuit is very simple. The circuit is rst tuned to resonance bycondenser 4 and without the object, and the output control of thegenerator A adjusted so that the pointer I3 of the galvanometer willindicate full scale reading.` The o bject is then placed between theelectrodes and the circuit again tuned to resonance. 'I'he pointer ofthe galvanometer is thereby deflected to indicate a certain value. Thepointer I8 of the resistance is then moved until it indicates thecorresponding value mark on dial I'I that was indicated by thegalvanometer. T he adjustment of the resistance changes the sensitivityof the galvanometer proportional to the loss conductance 1 R1 introducedby the object, and causes the galvanometer pointer to move to a readingon its dial I2a, which directly indicates the power absorbed by theobject. Any desired value of powe absorption might be administered tothe object by l varying the output control of the generator, thusvarying the value of the high frequency voltage across the circuit B.Correct values of the power absorption by the object will be indicatedregardless of the amount of output developed in the output circuit. Theresistance I4 is thus set once by means of the scale I1 for any run;that- In Fig. 2 of the drawing there is shown another form ofinstrumentality. An electronic tube E is bridged across the conductors Iand 2 of the output circuit and is connected with the moving coil 20 ofa galvanometer F of the dynamometer ty'pe. The excitation coil 2| of thegalvanometer is in the circuit 22 which includes the variable resistance23 so that the sensitivity of said galvanometer can be varied byadjusting the resistance and thereby the exciting current.

'Ihe inyention has been described in` connection with an indicatingdevice which utilizes the resonant voltages of the circuit, but it is tobe understood that indicating devices may be used which utilize resonantcurrent in the circuit as the motivating factor.

I claim:

1. In an apparatus for the determination of the power absorbed by theload in an oscillatory circuit, the combination of an electrical devicewith a square law characteristic, including a meter and a dialassociated therewith connected across the circuit thereby adapted toindicate a value proportional to the square of the voltage of saidcircuit, a variable resistance in parallel with the meter of said devicefor varying the sensitivity thereof and thereby operating upon saidvalue with a ratio factor to produce a final value indicated upon saidmeter proportional to the power absorbed by said load, a scaleassociated with said resistance for indicating the portion thereofshunted, said meter dial being calibrated to indicate the second valuein power units, said resistance scale having been calibrated tocorrespond with the percentage no load voltage damping caused by saidload, the insertion of the amount of said resistance thereby scaledbeing that which will cause the sensitivity of said meter to beinversely proportional to the equivalent loss resistance which would beintroduced into Said circuit by said load regardless of power absorbedthereby.

2. In an apparatus for the determination of the power absorption in aload in an oscillatory circuit, the combination of a hot wirethermocouple with a galvanometer for measuring the voltage across thecircuit, and indicating a value corresponding to the load component, thepower absorption of which is to be measured, a calibrated resistance inparallel with said galvanometer for changing the sensitivity thereof indenite relation to said load component and the corresponding value beingindicated by said galvanometer, said relation being dened by the formulal Rl corresponds to the sensitivity of said galvanometer, Pi being thepower absorbed by the load. E corresponding to the voltage across thecircuit and R1 corresponding to the equivalent parallel resistanceinserted into said circuit by said load.

3. In an apparatus for the determination of power absorption in a loadin an oscillatory circuit, the combination of an electrical device witha square law characteristic, a variable resistance in parallel with saiddevice, and a calibrated scale in connection with said variableresistance for adjusting the sensitivity of said electrical device to avalue proportional to the equivalent loss conductance introduced intothe circuit by the load, the power absorption of which is to bemeasured.

4. The method of indicating upon an electrical measuring instrument thepower absorbed by a load in a high frequency output circuit, includingthe steps of impressing upon the instrument a voltage proportional tothe square of the resonant no-ioad voltage of the circuit, inserting theload into the circuit and thereby adding an equivalent loss conductanceinto the circuit to cause a damping of the voltage therein to make acorresponding change in the voltage impressed on the instrument, andthen adjusting the sensitivity of said instrument so as to render thesame proportional to the equivalent loss conductance inserted into saidcircuit by said load; said instrument thereby reading a quantityproportionai to the value of the power absorbed.

EUGEN NUTTELIVIANN.

CERTIFICATE OF C ORRECTI ON Patent, No. 2,240,955. my 6, 19141.

EUGEN MITTELMANN.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correotion' as follows: `Page 2,first column, line Z-Zlg., for "crsuit" read -circuit; page 5, secondcolumn, line 56, for "criteron" read -criterion; page b, first column,line 52, for "l R1" read --R-- and that the said Letters Patent should.ve read with this correction therein that the seme may conform to therecord of the case in the Patent Office.

signed and sealed this 15th dew of July, A. D. 1914i.

Henry Van Arsdale,

(Seal) Acting Connnis sioner of Patents.

